The present invention relates to birefringent optical devices and methods.
The demand for telecommunication transport services has increased over recent years, primarily due to the increase in information transfer between computers. New multimedia applications and smart appliances are expected to increase demands on telecommunication networks in the coming years. Telecommunications service providers have sought to meet future demand by installing optical telecommunications systems. Optical telecommunications systems use light to carry information. Light advantageously can be used to carry large volumes of information at very high speeds.
Some optical transmission systems use wavelength division multiplexing (WDM) to increase the volume of information they can carry. WDM systems transmit multiple colors or wavelengths of light simultaneously to carry information, much like different radio channels carry different radio programs. Each wavelength or color of light represents one channel. To further increase the amount of information that can be transferred, the channels in some WDM systems are spaced very close together, for example, 50 GHz. Even closer channels spacings of 12.5-25 GHz are forecasted.
To receive or route optical signals in the WDM system, optical equipment is needed to distinguish between the different optical transmission channels, much like a radio tuner distinguishes between signals from different radio stations. An optical device capable of distinguishing between optical channels in a WDM system is an optical filter. For example, one known optical filter is a Solc-type filter made up of one or more high order birefringent waveplates stacked between optical polarizers. The optical filter can be designed to have periodic passbands. Frequencies of an optical signal located within the passbands pass through the filter. Frequencies outside of the passbands are cut off. The birefringent waveplates are used to shape and tune the passband(s) to particular frequencies. In particular, the position of the passbands of the optical filter depends, among other things, on the refractive index difference xcex94n of the birefringent material that forms the waveplates and the length l of the birefringent waveplates. The key to optical filter operation is to allow optical signals of selected optical channels to pass without significant losses, but to cut-off optical signals outside of the selected optical channels.
As the number of optical channels used in WDM systems increases, the passbands for optical filters need to be narrower and more precisely tuned. Several problems arise. First, both refractive index difference xcex94n and the length l of the waveplates change with temperature. Therefore, as temperature changes, the passbands of the optical filter shift. As a result, the optical filter may cut off or distort optical signals that should be passed and/or pass optical signals that should be cut off. In addition, it is difficult and expensive to manufacture waveplates with a highly precise length l. Even a slight variation in the length of the waveplates affects the location of the passband of the optical filter.
The present invention provides a stable, economical birefringent optical device capable of precision operation. The birefringent optical device may be used in an optical filter, may be part of WDM optical equipment, such as an add/drop device, dispersion compensator, a receiver, or other optical equipment.
The present invention also provides a method for manufacturing a stable birefringent optical device capable of precision operation.
The present invention also provides a method for tuning a birefringent optical device.